Mobile computing devices such as cellular telephones (cell phones) and personal digital assistants (PDAs) demand very high functional performance in relatively small packaging. Similar demands are placed on the aesthetics and appearance of such devices. An optical keypad is one component within a mobile computing device that requires both high functionality and appearance. To provide a high quality appearance and increased functionality, many keypads are backlit so that the characters on the individual keys are illuminated and easy to read. There are two basic forms of backlighting: direct and indirect.
Conventionally, direct backlighting implements several light emitting diodes (LEDs) mounted directly behind the keypad. These LEDs emit light directly towards the back of the keys, so that a portion of the emitted light passes through translucent portions (e.g., the characters) of the keys. While direct backlighting can provide good backlighting for the keypad, direct illumination is typically expensive because many LEDs are distributed behind the keypad. Also, direct backlighting typically consumes a significant amount of power because of the number of LEDs that are used to generate the backlight illumination.
In contrast to direct backlighting, conventional indirect backlighting uses one or more LEDs mounted at an edge of a light guide behind the keypad. The LEDs emit light into the light guide, which transfers the light through total internal reflection (TIR) across the length and/or width of the keypad. In general, TIR keeps all of the light inside the light guide, so that the light can travel across the length and/or width of the keypad, as long as the light is reflected at relatively large angles (i.e., angles of incidence which are larger than the critical angle, as measured from the surface normal) within the light guide. However, there is a balance between facilitating TIR and allowing some light to escape from the light guide in order to provide backlight illumination for the keypad. If all of the light were to reflect inside the light guide through TIR, then there would be no light to illuminate the keypad. Conversely, if none of the light reflects inside the light guide, then the light would not reach the far side of the light guide and the keypad, and the backlight illumination of the keypad would be imbalanced with bright spots close to the LEDs and dark spots away from the LEDs. Hence, some of the light inside the light guide should be internally reflected, while the remaining light uniformly exits the light guide at the various key locations.
Both direct and indirect backlighting arrangements can suffer from inadequate illumination. In particular, some arrangements result in relatively low brightness because of limitations in the beam distribution pattern of the LEDs. In other words, the LEDs have limited beam distribution patterns, which makes uniform backlighting difficult to achieve, especially in indirect backlighting arrangements where the LEDs are located around the perimeter of the light guide and keypad.
One conventional way to improve the light distribution uniformity is to use surface feature patterns which are aligned with the individual keys of the keypad. The surface feature patterns are typically groups of raised or depressed surface features which cause the light to scatter in an approximately diffuse manner. However, such surface feature patterns can be insufficient to provide sufficient uniformity, especially for areas that are outside of the limited beam distribution pattern of the LEDs.
Another conventional way to increase light distribution uniformity is to add an adhesive to the surface of the light guide, similar to a surface feature pattern. However, adding adhesives can increase the cost of production of the device. In particular, the process of applying adhesives is not suitable for mass production. Also, it is difficult using adhesives to control the consistency of the brightness within the light guide.
Another conventional way to increase light distribution uniformity is to add serrations in front of the LEDs. Specifically, the serrations present a non-planar surface for the light to enter the light guide. The non-planar nature of the serrations causes the light to distribute the light across a wider angle within the light guide, because the serrations direct some of the light toward the near corners (i.e., dark zones) of the light guide. However, consistently implementing serrations in the proper locations is difficult because it is hard to control the serration tooling after running for some period. In particular, the shape of the serration tooling easily wears over time due to the small and irregular size of the serration tooling.
Another conventional way to increase light distribution uniformity is to use higher intensity LEDs, or to include more LEDs. However, these solutions increase the cost of production of the device.